1. The Field of the Invention
The present invention is directed generally to heat exchangers, and more particularly, to a coil tube heat exchanger for a tankless hot water system.
2. Background Art
In a tankless hot water system, a buffer tank is typically used as a limited energy storage device to meet sudden high hot water demand and to lessen the effects of temperature spike. In prior art tankless hot water systems having at least a fin-and-tube heat exchanger, a separate storage device is commonly used as the buffer tank and disposed externally from the fin-and-tube heat exchanger. The fin-and-tube heat exchanger comprises a generally cylindrical housing, a helix coil disposed concentrically inside the housing, a radial-fired burner disposed inside the coil lumen on one end of the helix coil and a ceramic disc disposed inside the helix coil lumen on the opposite end of the helix coil. Typically a top casting fixedly disposed on top of the housing serves as an interface between a fan blower which forces an air/fuel mixture flow to the burner. The ceramic disc serves as a barrier to shield hot flue gas from damaging components in its path and to channel hot flue gas to more effectively surround the helix coil external surfaces to improve heat transfer from flue gas to the water flowing inside the helix coil. However, the use of a ceramic disc inside the lumen takes up valuable heat exchanger footprint, increases fabrication and installation costs and fails to harness and recover the maximum amount of energy. In such installations, typically fluid baffle plates are used and positioned between coil windings (loops) such that hot flue gas can be more efficiently directed around coil tube. Though effective in enhancing heat transfer from the hot flue gas to the helix coil, there remain gaps in the path of the hot flue gas to escape through. Poor heat recovery through the top casting further causes an unnecessarily warm top casting, waste to the environment and unnecessarily heats up surrounding components.